Tandem solar cells comprising stacked unit solar cells with different band gaps are known as devices for providing improved photoelectric conversion efficiency by utilizing its wide wavelength range. An example of such a tandem solar cell is disclosed in Japanese Patent Laid-Open Publication No. Hei 4-226084.
FIG. 15 is a cross sectional view showing a conventional tandem solar cell. Referring to FIG. 15, a solar cell 10 includes a top cell 12 which is a unit solar cell provided on the light incident side (top of the figure), a bottom cell 14 which is a unit solar cell provided on the back side, and a tunnel diode 16 provided between these cells. The solar cell also includes a top electrode 18 disposed on the light incident side, and a bottom electrode 20 on the back side. Generally, in such a tandem solar cell, a solar cell with a wide band gap (Eg) is used for the top cell, while a solar cell with a narrow band gap is used for the bottom cell.
FIG. 16 shows a band structure of the solar cell taken along the broken line D in FIG. 15. As shown in FIG. 16, carriers, i.e. electrons and holes, cannot be transported through the junction between the top and bottom cells 12 and 14 because of the difference between the band levels of the cells 12 and 14. Therefore, the tunnel diode 16 is provided at the junction for permitting movement of carriers at this junction, so that the top cell 12 and the bottom cell 14 are connected in series to function as an integrated solar cell. Because the top and bottom cells 12 and 14 can absorb light with different wavelengths from each other, the above-described structure can absorb light with a wider range of wavelengths, to thereby enhance photoelectric conversion efficiency.
In the conventional solar cell 10 in FIG. 15, however, the currents flowing through the top and bottom cells 12 and 14 must be equal to each other because the cells 12 and 14 are connected in series. Therefore, the top and bottom cells 12 and 14 of the solar cell 10 shown in FIG. 15 must have a thickness such that the same amount of carriers are generated in these cells and the same amount of currents flow therethrough. As a result, it is not possible to provide the cells with an optimum thickness for high photoelectric conversion efficiency.
Further, considerable resistance loss and carrier recombination loss are observed in the tunnel diode 16 provided between the top and bottom cells 12 and 14.
In addition, the solar cell suffers from a high defect density due to lattice mismatch at the interface between the top and bottom cells 12 and 14, leading to lose of minority carriers due to recombination at this portion.
The present invention has been conceived in view of the above problems, and aims to provide a solar cell having a thickness most advantageous for a high photoelectric conversion efficiency and capable of reducing carrier recombination loss.